The present invention relates to a process for producing syndiotactic 1,2-polybutadiene and a catalyst composition for use therein.
Syndiotactic 1,2-polybutadiene is a thermoplastic resin that has a stereoregular structure in which the vinyl groups as side chains are located alternately on the opposite sides in relation to the polymeric main chain consisting of carbon-carbon bonds. Syndiotactic 1,2-polybutadiene is a unique material that combines the properties of plastics and rubber. Accordingly, syndiotactic 1,2-polybutadiene has many uses. For example, films, fibers and molded articles can be made utilizing syndiotactic 1,2-polybutadiene. It can also be blended into rubbers and cocured therewith.
Syndiotactic 1,2-polybutadiene can be made by solution, emulsion or suspension polymerization. The syndiotactic 1,2-polybutadiene from solution, emulsion or suspension polymerization typically has a melting temperature that is within the range of about 195.degree. C. to 215.degree. C. However, for processability reasons it is generally desirable for syndiotactic 1,2-polybutadiene to have a melting temperature of less than about 195.degree. C. to render it suitable for practical utilization.
Various transition metal catalyst systems based on cobalt, titanium, vanadium, chromium, and molybdenum have been reported in the prior art for the preparation of syndiotactic 1,2-polybutadiene (see, e.g., L. Porri and A. Giarrusso, in Comprehensive Polymer Science, edited by G. C. Eastmond, A. Ledwith, S. Russo and P. Sigwalt, Pergamon Press: Oxford, 1989, Volume 4, Page 53). However, the majority of these catalyst systems have no practical utility because they have low catalytic activity or poor stereoselectivity and in some cases produce low molecular weight polymers or crosslinked polymers unsuitable for commercial use.
The following two catalyst systems based on cobalt-containing compounds are well known for the preparation of syndiotactic 1,2-polybutadiene on a commercial scale: (1) cobalt bis(acetylacetonate)/triethylaluminum/water/triphenylphosphine (U.S. Pat. Nos. 3,498,963 and 4,182,813; Jap. Kokoku 44-32426, assigned to Japan Synthetic Rubber Co. Ltd.), and (2) cobalt tris(acetylacetonate)/triethylaluminum/carbon disulfide (U.S. Pat. No. 3,778,424; Jap. Kokoku 72-19,892, 81-18,127, 74-17,666, and 74-17,667; Jap. Kokai 81-88,408, 81-88,409, 81-88,410, 75-59,480, 75-121,380, and 75-121,379, assigned to Ube Industries Ltd.). These cobalt-based catalyst systems also have serious disadvantages.
The cobalt bis(acetylacetonate)/triethylaluminum/water/triphenylphosphine system yields syndiotactic 1,2-polybutadiene having very low crystallinity. In addition, this catalyst system develops sufficient catalytic activity only in halogenated hydrocarbon solvents as the polymerization medium, and halogenated solvents present the problems of toxicity.
The cobalt tris (acetylacetonate)/triethylaluminum/carbon disulfide system uses carbon disulfide as one of the catalyst components. Because of its high volatility, obnoxious smell, low flash point and toxicity, carbon disulfide is difficult and dangerous to use and requires expensive safety measures to prevent even minimal amounts escaping into the atmosphere. Furthermore, the syndiotactic 1,2-polybutadiene produced with this catalyst system has a very high melting temperature within the range of 200-210.degree. C., which makes it difficult to process the polymer. Although the melting temperature of the syndiotactic 1,2-polybutadiene can be reduced by the use of a catalyst modifier as a fourth catalyst component, the presence of such a catalyst modifier also has an adverse effect on the catalyst activity and polymer yields. Accordingly, many restrictions are required for the industrial utilization of the two aforesaid cobalt-based catalyst systems of the prior art.
Coordination catalyst systems based on iron-containing compounds such as iron(III) acetylacetonate/triethylaluminum have been known in the prior art for a long time, but they have very low catalytic activity and poor stereoselectivity for the polymerization of 1,3-butadiene and sometimes give rise to oligomers, low molecular weight liquid polymers or crosslinked polymers. Therefore, these iron-based catalyst systems of the prior art have no industrial utility.
Under such circumstances, in order to overcome the above-mentioned disadvantages of the prior art, intensive research and investigations were made by the present inventor for the purpose of developing and providing a new and significantly improved catalyst composition, which is not restricted technically in industrial utilization and has high catalytic activity and stereoselectivity for the production of syndiotactic 1,2-polybutadiene having various melting temperatures.